In recent years, popularization of renewable energies, such as wind power generation, solar light power generation, and solar heat power generation, is accelerated, and in order to cover a further large amount of demanded power by renewable energies, examinations on wind power generation on the ocean, and solar light and solar heat power generations at desert regions have begun. In the case of the wind power generation on the ocean, it is necessary to, for example, transmit a large amount of generated power to cities where the power is to be consumed via undersea cables, and to highly efficiently transmit a large amount of power from a closed-off desert regions in Africa or China to Europe and a large city in a coastal area. As for such needs, a DC power transmission is highly efficient in comparison with a conventional three-phase AC power transmission, and thus examination on an establishment of a DC power transmission network has begun since such a network can be installed with suppressing costs.
In the case of DC power transmission, power converters are needed, such as a converter that converts a generated AC power into a DC power for DC power transmission, and an inverter that converts the transmitted DC power into an AC power for a city. Recently, a Modular Multilevel Converter (MMC) is already in practical use, MMC is capable of outputting a voltage waveform resembling a sine wave so as not to cause harmonic originating from the switching operations by the converter and inverter to flow to the AC system, and is capable of reducing the number of output filters.
FIG. 10 is a circuit diagram illustrating a unit block that forms a conventional MMC.
A chipper cell C that is a unit block has a leg 1 and a capacitor (c_ch) 2 connected in parallel. The leg 1 includes two switching element (sw_ch1) 3a and switching element (sw_ch2) 3b connected in series.
FIG. 11 illustrates an example MMC utilized for a conventional DC power transmission application. An MMC 50 has a U-phase leg 51, V-phase leg 52, and a W-phase leg 53 connected in parallel and to a DC power supply 54. Each leg is connected to a three-phase transformer (tr) 55, and this three-phase transformer (tr) 55 is connected to a power system (V_S) 56. Each leg includes the above-explained 12 chopper cells C connected in series. In addition, provided at the center of the U-phase leg 51 are a reactor (lb_up) 57a and a reactor (lb_un) 57b, provided at the center of the V-phase leg 52 are a reactor (lb_vp) 58a and a reactor (lb_vn) 58b, and, provided at the center of the W-phase leg 53 are a reactor (lb_wp) 59a and a reactor (lb_wn) 59b. 
As for the operation of this MMC 50, an explanation will be given of the U-phase leg 51 as an example. A total voltage v_up of positive chopper cells ch_up 1 to 6 is subtracted from an input DC voltage V_dc of the DC power supply 54, and a total voltage v_un of negative chopper cells ch_un 1 to 6 is added with reference to a reference voltage, and thus an AC voltage is obtained. In addition, this AC voltage is converted into a desired AC voltage by the three-phase transformer (tr) 55. Still further, the reactor (lb_up) 57a and the reactor (lb_un) 57b suppress an increase in current due to a short-circuit between the input DC voltage v_dc and a chopper cell output voltage v_up+v_un. The same is true of the V-phase leg 52 and the W-phase leg 53. A three-phase AC voltage is generated through the above-explained operation.